1. Field of the Invention
The present invention relates to a monolithic varistor, more particularly, to a monolithic varistor which comprises ZnO as a primary component and has a varistor voltage of 100 V or more. The present invention also relates to a ceramic for producing the varistor and to a method for producing the varistor. Throughout the specification, "varistor voltage" refers to voltage across the varistor measured at a current of 1 mA.
2. Background Art
In recent years, development of a chip-type element and employment of higher frequencies have progressed along with the trend of miniaturization of electronic devices and higher-speed circuit operation. In addition, such an element is required to have a reduced size, especially in terms of height, in order to increase the packaging density of a circuit. A non-linear resistor, i.e., varistor serving as a noise-absorbing element, is not an exception; a chip-type varistor which is formed of a ceramic predominantly comprising zinc oxide or strontium titanate has brought on the market. In contrast, a single-layer varistor having lead terminals or a varistor in which a single varistor layer is "molded-in" a resin or glass has been used as a varistor having a high varistor voltage such as a varistor for alternating current.
However, the conventionally employed single-layer varistor has a drawback that when the maximum peak current is desired to be increased, the electrode area must also be enlarged, thus failing to attain miniaturization of the varistor; whereas miniaturization of the varistor is possible only at the cost of maximum peak current. Thus, miniaturization of a varistor having a varistor voltage of 100 V or more has seen no progress. To cope with the dilemma, a monolithic ceramic varistor comprising a layered ceramic body in which a plurality of internal electrodes are formed is desirable. In this case, however, the varistor voltage per unit thickness thereof must be increased. To this end, the grain size of the ceramic must be reduced without lowering the maximum peak current per unit area.